The present invention is based on Japanese Patent Application No. 2000-285365, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a stepping motor having a structure which is free from the declination of the pole teeth of the stator cores, and a method of manufacturing the same.
2. Description of Related Art
Various types of stepping motors have been proposed. In each of those, a rotor consists of a permanent magnet magnetized such that N poles and S poles are alternately arranged in the hoop direction. Stators are disposed on the outer peripheral surface of the rotor. Each stator contains a coil bobbin, which integrally contains the stator cores as the result of resin molding and is used for supporting a coil wound thereon. The stepping motor thus constructed suffers from such a problem that when the stator cores are made integral with the coil bobbin by resin molding, the pole teeth are declined outside by a resin molding pressure. When the pole teeth are declined outside, a gap between the pole teeth and the rotor is excessively large, and the increase of the gap greatly affects the motor characteristics. To avoid this problem, the molding pressure is carefully adjusted so that the pole teeth are not declined outside. This forms one of the major factors to increase the cost to manufacture.
A stepping motor disclosed in JP-A-6-165467 includes means to prevent the pole teeth from being declined outside. The stepping motor is of the 2-phase type. Specifically, as shown FIG. 5, two core sets each consisting of two stator cores 41a and 41b (41c and 41d) are superimposed in a 2-phase fashion. In each core set, only the spaces among the pole teeth are filled with synthetic resin 81 by one piece molding. The synthetic resin 81 is prohibited from flowing into the outside of the pole teeth. In the structure, the molding pressure of the synthetic resin 81 does not affect the pole teeth in the radial direction, so that the pole teeth are not declined outside. In the stepping motor thus constructed, a coil (not shown) covered with an insulating film is directly wound on the outer peripheral surfaces of the pole teeth.
In the stepping motor disclosed in JP-A-6-165467, as described above, the stator cores 41a, 41b, 41c and 41d of the core sets are joined together by merely filling the spaces among the pole teeth with the synthetic resin 81. Consequently, a holding force for integrally holding the stator cores 41a, 41b, 41c and 41d, viz., a rigidity of the coil bobbin after molded, is weak. Further, the coil is wound in a state that it is in direct contact with the outer peripheral surfaces of the pole teeth. This creates an insulating problem of the coil insulated, specifically, the coil is covered with an insulating film. In winding the coil, the insulating film of the coil can be broken. As a result, there is a fear that the conductive wire (copper wire) of the coil is exposed outside at the part of the coil where the insulating film is broken. Contact of the conductive part or wire of the coil with the pole teeth results in short-circuiting. To apply insulating films to the pole teeth is one of solutions to this problem. This solution, however, involves other problems of increasing the number of manufacturing steps and the cost to manufacture.
Accordingly, an object of the present invention is to provide a stepping motor which prevents the pole teeth from being declined in the radial direction, secures a rigidity of the coil bobbin integrally including the stator cores, and solving the short-circuiting problem and others.
To achieve the above object, there is provided a stepping motor having a rotor consisting of a permanent magnet magnetized such that N poles and S poles are alternately arranged in the hoop direction, stator cores with pole teeth oppositely disposed on the outer peripheral surface of the rotor, and a coil bobbin integrally containing the stator cores as the result of resin molding and being used for supporting a coil wound thereon. The stepping motor is improved such that holes are formed in the coil bobbin, and the holes range from an outer wall of the coil bobbin to the outer peripheral surfaces of the pole teeth.
With provision of holes each ranging from an outer wall of the coil bobbin to the outer peripheral surfaces of the pole teeth, it is possible to prevent the pole teeth from declination in the radial direction in the stage of a resin molding, and to secure a rigidity of the coil bobbin in a certain level. In the resin molding, the molding die is provided with protrusions to be located within the holes, and the tops of the protrusions of the die are brought into contact with the outer peripheral surfaces of the pole teeth, and in this state, the resin injection is performed. As a result, the pole teeth are prevented from being declined outside. The coil bobbin includes the holes continuous to the pole teeth. However, provision of those holes do not lead to reduction of a rigidity of the coil bobbin. Additionally, the coil does not directly contact with the outer peripheral surfaces of the pole teeth, but is wound around the outer peripheral surface of the coil bobbin. Even if the insulating film of the coil is broken, there is no fear that the insulating film of the coil is broken, and its conductive part exposed comes in contact with the pole teeth.
In another aspect of the stepping motor constructed according to the invention, the width of each the hole as viewed in the hoop direction is narrower than the width of each the pole tooth at a position corresponding to each the hole. The rigidity of the coil bobbin is further increased. Further, there is no fear that the coil wound on the coil bobbin is loosened, and comes in contact with the pole teeth located at the inner part of the holes.
In yet another aspect of the stepping motor of the invention, a plurality of the holes are provided, and an inner side wall as viewed in the hoop direction of each the hole is perpendicular to a predetermined imaginary line including the center of the stepping motor. Accordingly, the die used for resin molding the coil bobbin of the stepping motor may be constructed such that it is vertically parted into two parts along the predetermined imaginary plane. Therefore, the die is simplified in construction, and the cost to manufacture is reduced.
In still another aspect of the stepping motor, the outside diameter of the stepping motor is 20 mmxcfx86 or less. Usually, of the stepping motors having the outside diameter of 50 mmxcfx86 or smaller, called small motors, those motors of 20 mmxcfx86 in outside diameter are thin in its stator core, and hence the pole teeth are easy to be declined by the molding pressure during the molding process. Further, the space used for coil winding is small, and the number of coil turns is small, when comparing with those in the small stepping motor. The declination of the pole teeth less affects the motor characteristics. The invention is free from such a problem since there is no case that the pole teeth are declined.
According to another aspect of the present invention, there is provided a method of manufacturing a stepping motor having a rotor consisting of a permanent magnet magnetized such that N poles and S poles are alternately arranged in the hoop direction, stator cores with pole teeth oppositely disposed on the outer peripheral surface of the rotor, and a coil bobbin integrally containing the stator cores as the result of resin molding and being used for supporting a coil wound thereon. In the manufacturing method, a cylindrical core member is disposed on the inner peripheral surfaces of the pole teeth. At least two outside dies each having protrusions protruded toward the core member are disposed on the outer peripheral surfaces of the pole teeth. The protrusions are brought into contact with the outer peripheral surfaces of the pole teeth, thereby preventing the pole teeth from declination, and in this state, cavities present between the core member and the outside dies are filled with resin, whereby the resin is made integral with the stator cores to form the coil bobbin.
The radially outward declination of the pole teeth by the molding pressure is prevented in a manner that the resin injection is carried out while the protrusions of the outside die are brought into contact with the outer peripheral surfaces of the pole teeth of the stator cores. In the coil bobbin thus molded, after the die having the protrusions is die cut, the holes ranging from the outer wall to the outer peripheral surfaces of the pole teeth are formed. However, formation of those holes does not lead to reduction of a rigidity of the coil bobbin. Additionally, the coil is wound on the outer peripheral surface of the coil bobbin made of resin, viz., it is not in direct contact with the outer peripheral surfaces of the pole teeth. With this, even if an insulating film on the coil surface is broken, there is no fear that the exposed metal part comes in contact with the pole teeth, resulting in short-circuiting.
In another method of manufacturing the stepping motor, each protrusion is narrower than the width of the same at a position where the protrusion comes in contact with the pole tooth. Accordingly, the width of the hole formed by each protrusion as viewed in the hoop direction is narrow, so that the coil bobbin rigidity integrally including the stator cores by resin molding is further increased. Further, there is no fear that the coil wound on the coil bobbin is loosened, and comes in contact with the pole teeth located at the inner part of the holes.